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j-CarrageenanSodium Alginate Beads and Superabsorbent Coated Nitrogen Fertilizer with Slow-Release, Water-Retention, and Anticompaction Properties Yanfang Wang, Mingzhu Liu,* Boli Ni, and Lihua Xie State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, and Department of Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of China. ABSTRACT: A multifunctional slow-release nitrogen fertilizer (SRNF) has been developed to improve fertilizer use efficiency and reduce environmental pollution. k-Carrageenansodium alginate (kCSA) and cross-linked kC-g-poly(acrylic acid)/Celite superabsorbent were used as inner and outer coating materials, which were coated consecutively on the granule core urea in a pan granulator. Elemental analysis result showed that the product contained 22.6% nitrogen. The water evaporation as well as the nitrogen release behavior of SRNF in soil was explored. In addition, the coated urea had a higher average crushing strength in comparison with uncoated urea. Furthermore, the incorporation of SRNF in soil could effectively prevent the compaction of soil. These studies showed that the product prepared by a simple route with good slow-release and water-retention properties may be expected to have wide potential applications in modern agriculture and horticulture.
1. INTRODUCTION Modern agriculture now feeds 6000 million people,1 and fertilizer is a vital input material for the sustainable development of crop production and food security.2 Worldwide experiences in agricultural development have proved that rational fertilization is one of the most efficient and important methods to increase crop yields. Fertilizer consumption in China is also challenging the acceptance limit of resource and environment. From 1977 to 2005, annual synthetic nitrogen (N) fertilizer application increased from 7.07 to 26.21 million tons (a 271% increase) in China.3 However, compared with 45% in developed countries, the degree of utilization of N fertilizer in China is only in the range of 3035%. Meanwhile, nitrate leaching and groundwater contamination have been shown to be related to N fertilizer use in agriculture. Additionally, some recent studies have shown that overuse of N fertilizer has already induced serious environmental hazards, including soil acidification, heavy metal contamination, and greenhouse gas emission.4 Among the N fertilizers, the most widely used one is urea, because of its high nitrogen content (46%) and comparatively low cost of production. After being applied to soil, it can be rapidly hydrolyzed to NH3 and CO2 by soil urease, followed by NO3 formation through nitrification. Therefore, ammonia loss and nitrate leaching are environmental concerns in regions where urea is applied.5 When concerns about the sustainability of input-intensive agriculture and the economic, ecological, and environmental effects of N fertilizer overuse are taken into consideration, it is clear that use of slow-release fertilizers (SRFs) is one attempt to enhance the efficiency of fertilizer to mitigate these problems.6 SRFs allow the active component to slowly diffuse toward the soil, thus making it available in the field for a longer period of time.7 In case of urea, slow-release products are typically made by reacting urea with various aldehydes to reduce the solubility of the material. Another method of regulating N fertilizer release is the application of a coating. Polymer-coated SRFs look promising for r 2011 American Chemical Society
widespread use in agriculture because the science and technology of polymers has undergone explosive growth during recent years and they can be designed to release nutrients in a more controlled manner.8 The current trend of using environmentally friendly products has resulted in an extensive exploitation of new materials obtained from renewable resources. Preparation of polysaccharide-based composites, blends, or mixtures represents a new category of environmentally safe materials for newer applications, due to their water solubility, renewability, nontoxicity, and biodegradability.9,10 Sodium alginate (SA) is an anionic natural macromolecule, which is composed of poly-β-1,4-D-mannuronic acid and α-1,4-L -guluronic acid in varying proportions by 14 linkages. It can be extracted from marine algae or produced by bacteria.11 k-Carrageenan (kC) is a negatively charged polysaccharide extracted from red seaweed that has a linear sulfated backbone of alternating α-1,4- and β-1,3-linked galactose residues.12 SA together with kC was used in the drug delivery systems and special membranes.1315 Their joint use as coating material in agriculture has not been reported. Most recently, we prepared kCSA complex beads by dropping an aqueous solution of kC and SA into calcium chloride (CaCl2) and potassium chloride (KCl) mixed solution to form ionically cross-linked hydrogel beads, which were used as a fully natural coating material. Mixing of kC with SA could improve its mechanical properties and reduce brittleness of the hydrogel.16 In addition, there is a need to explore the utilization of abundantly available seaweed polysaccharides. Besides fertilizer, water is another important factor that limits the production of agriculture. China is one of the world’s most Received: September 8, 2011 Accepted: December 21, 2011 Revised: December 15, 2011 Published: December 21, 2011 1413
dx.doi.org/10.1021/ie2020526 | Ind. Eng. Chem. Res. 2012, 51, 1413–1422
Industrial & Engineering Chemistry Research water-deficient economies, and the scarcity of water is viewed as a major threat to long-term food security. Therefore, efficient management of soil moisture is important for agricultural production in response to scarce water resources. Superabsorbents are slightly cross-linked hydrophilic polymers that can absorb huge volumes of water without dissolving or losing their three-dimensional structures and can retain the absorbed water even under certain pressure.17 Because of their excellent characteristics of water retention and absorbency, superabsorbents had been used in agriculture and horticulture since their advent in 1969.18,19 Recently, several studies2022 have confirmed the importance of superabsorbents in improving physical properties of the soil such as porosity, structure, and water-holding capacity. Johnson23 reported 171402% increases in water retention capacity when superabsorbents were incorporated in coarse sand. Islam et al.4 reported that the application of superabsorbents could conserve soilwater, making it available to plants for increased biomass accumulation and reduced oxidative stress, especially under severe water stress. Similarly, plant height, leaf area, and number of grains as well as protein, soluble sugar, and starch contents in the grain also increased with superabsorbent treatment. Mikkelsen et al.24 found that addition of superabsorbent to the fertilizer solutions reduced N leaching losses from soil columns by as much as 45% during the first 4 weeks in heavily leached conditions compared with N fertilizer alone. Moreover, the use of superabsorbent materials as carrier and regulator of nutrient release was helpful in reducing undesired fertilizer losses while sustaining vigorous plant growth.25 To satisfy these requirements, we prepared a double-coated slow-release and water-retention nitrogen fertilizer. Its core is urea fertilizer granule, the first coating layer is kCSA bead, and the second coating layer is kC-g-poly(acrylic acid)/Celite superabsorbent. The choice of the coatings is largely dictated by the problems to be addressed above. By doing so, first, superabsorbents were used as the outer coating instead of blending or polymerizing with fertilizers; this process reduced the loss of fertilizer without altering the properties of water retention and absorbency. Second, the kCSA layer was incorporated into the fertilizer production to make the products cheaper and easier to biodegrade.2629 Meanwhile, hydrophilic groups of kCSA matrix were cross-linked completely by potassium and calcium ions. This approach turned hydrophilic kC and SA into the hydrophobic kCSA bead, which contributed to the slowrelease behavior of fertilizer. In addition, the coated fertilizer is expected to retard nitrogen release, improve soil moisture, reduce the use of water, and alleviate environmental hazards caused by excessive fertilization. Therefore, the main purpose of this study was to determine the release characteristics of the coated fertilizer and its effect on improving the water-holding capacity of soil. Research has been ongoing to prepare a multifunctional fertilizer that can be used in agro-industries.
2. EXPERIMENTAL SECTION 2.1. Materials. The source of nitrogen used was commercial pelleted urea, which was previously sieved to be between 2.0 and 2.5 mm in diameter. Sodium alginate (SA; the viscosity of a 2% solution is 3200 mPa 3 s at 25 °C) was obtained from Qingdao Haiyang Chemical Co. (Qingdao, China). k-Carrageenan (kC) was purchased from the Golden Phoenix of k-Carrageenan Co. Ltd. (Tengzhou, China) and used without further purification. Acrylic acid (AA, chemical grade; Beijing Eastern Chemical
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Works, Beijing, China) was distilled at reduced pressure before use. N,N0 -Methylenebis(acrylamide) (NNMBA) was recrystallized from 95% ethanol prior to use. Ammonium persulfate (APS) was recrystallized from distilled water before use. Celite, supplied by Shanghai Chemical Reagent Factory (Shanghai, China), was calcined at 450 °C, and particle sizes
